There are various proposed image display devices configured to display an image by means of laser beams generated by laser sources such as laser diodes. An electronic printing device (laser printer or copier), which scans a beam to form an image, is exemplified as the image display device. A photo-plotter, which directly irradiates recording paper with red, green and blue laser beams to burn-in a photo, is exemplified as another image display device. A projector, which irradiates a screen, instead of the aforementioned recording paper, with red, green and blue laser beams in order to display an image, is exemplified as yet another image display device. Another projector, which is used as an image display device, irradiates a two-dimensional modulator such as an ultra-small liquid crystal element or digital mirror device to display an image.
Semiconductor laser diodes (LD) and semiconductor light emitting diodes (LED) are exemplified as laser sources used for the aforementioned various image display devices. It is known that temperature rises of light emitting elements such as semiconductor laser diodes (LD) and semiconductor light emitting diodes (LED) shift the central wavelength of the emission spectrum toward a longer wavelength range or change the spectrum shape itself. It is also known that specific semiconductor laser sources and light emitting diodes change the central wavelength or spectrum shape in response to the input current value.
Patent Document 1 discloses an LED drive method for suppressing the emission spectrum variations during adjusting LED brightness of the projector. According to Patent Document 1, several light quantity control modes are switched in response to analysis results about color components of image signals.
Patent Document 2 discloses a method for preventing the temperature rise of the light source from shifting the central wavelength of the emission spectrum to a long wavelength range. According to Patent Document 2, the light source is pulse-modulated. A ratio of a turn-on period to a turn-off period of the laser source is adjusted to suppress the temperature rise of the light source. As a result, it becomes less likely to shift the wavelength.
Patent Document 3 discloses a head-mounted display provided with a hologram mirror. The disclosed technologies in Patent Document 3 aim to decrease variations in diffraction efficiency and diffraction angle of the hologram mirror, which are caused by the wavelength shift that is induced by temperature changes of the laser source. Patent Document 3 teaches a method for dissipating heat from the light source to maintain a constant temperature of the laser source and a method for providing several different light sources in the central wavelength.
The disclosed technologies in the aforementioned Patent Documents 1 and 2 contain problems about control under actual usage conditions. Under the actual usage conditions, the wavelength shifts caused by temperature variations go along with input power variations. Therefore, it is difficult to suppress variations in the emission spectrum and prevent the central wavelength of the emission spectrum from shifting.
Variations in the current supplied to a laser source with a narrow spectrum band may widen the spectrum band. Therefore, it has not been verified whether or not only the wavelength shift caused by temperature is compensated by the input current.
Variations in wavelength shift may not be suppressed only by the temperature control of the light source suggested in the technologies disclosed in Patent Document 3. In addition, the disclosed technologies in Patent Document 3 require several different light sources in the central wavelength of the emission spectrum. Therefore, the disclosed technologies in Patent Document 3 are also problematic in terms of cost.    Patent Document 1: JP 2008-102442 A    Patent Document 2: JP 2009-99701 A    Patent Document 3: JP 2007-226190 A